Subtitles displayed by rendering a graphics stream are important means for people in different linguistic areas to enjoy foreign-language films. Such a graphics stream is multiplexed with a video stream that represents a moving picture, and recorded on a recording medium. The graphics stream includes a plurality of display sets that are each made up of display control information and graphics data. Each of the display sets is used for displaying an individual subtitle in reproduction of a film. The display sets are read from the recording medium and processed one by one as the reproduction of the moving picture progresses, to display the subtitles together with the moving picture.
Here, if each display set is processed only after processing of an immediately preceding display set is completed, a processing delay develops. Especially when each display set has a high resolution such as 1920×1080, a significant processing delay occurs. Thus, when the graphics stream contains multiple display sets, the need for parallel processing of display sets arises.
One display set may carry not only an object definition segment referenced by its own presentation composition segment but also an object definition segment referenced by a presentation composition segment of a succeeding display set. In such a case of transferring an object definition segment not referenced by the presentation composition segment, when a plurality of object definition segments are loaded to a reproduction apparatus, a boundary between object definition segments that belong to one display set and object definition segments that belong to a succeeding display set is ambiguous.
To perform complete parallel processing of display sets, however, a reproduction apparatus for recording media such as BD-ROMs needs to be equipped with a dual processor-controller system where two processors each decode graphics data and two controllers each control the decoding of the corresponding processor. This makes an internal construction of the reproduction apparatus more complex. If the internal construction is more complex, the manufacturing cost is higher, which hinders wider use of reproduction apparatuses. Thus, even if parallel processing can be achieved with a dual processor-controller system, such an architecture is detrimental to widespread use of reproduction apparatuses, and therefore undesirable in terms of standardization.